io: document WriteString calls Write exactly once

Fixes #13849

Change-Id: Idd7f06b547a0179fe15571807a8c48b7c3b78d7c
Reviewed-on: https://go-review.googlesource.com/21852
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/io/io.go b/src/io/io.go
index 023473c79bdb0a8d66b27b372b46c4b5e1d08831..c36ec2afbb0915ea28665cffc5d6c5b42f135369 100644 (file)
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -291,6 +291,7 @@ type stringWriter interface {
 
 // WriteString writes the contents of the string s to w, which accepts a slice of bytes.
 // If w implements a WriteString method, it is invoked directly.
+// Otherwise, w.Write is called exactly once.
 func WriteString(w Writer, s string) (n int, err error) {
        if sw, ok := w.(stringWriter); ok {
                return sw.WriteString(s)